1. FIELD OF THE INVENTION
The field of the invention is regeneration of coked cracking catalyst in a fluidized bed.
2. DESCRIPTION OF RELATED ART
Catalytic cracking is the backbone of many refineries. It converts heavy feeds into lighter products by catalytically cracking large molecules into smaller molecules. Catalytic cracking operates at low pressures, without hydrogen addition, in contrast to hydrocracking, which operates at high hydrogen partial pressures. Catalytic cracking is inherently safe as it operates with very little oil actually in inventory during the cracking process.
There are two main variants of the catalytic cracking process: moving bed and the far more popular and efficient fluidized bed process.
In the fluidized catalytic cracking (FCC) process, catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts a source of hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425.degree.-600.degree., usually 460.degree. C.-560.degree. C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 500.degree. C.-900.degree. C., usually 600.degree. C.-750.degree. C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Catalytic cracking is endothermic, it consumes heat. The heat for cracking is supplied at first by the hot regenerated catalyst from the regenerator. Ultimately, it is the feed which supplies the heat needed to crack the feed. Some of the feed deposits as coke on the catalyst, and the burning of this coke generates heat in the regenerator, which is recycled to the reactor in the form of hot catalyst.
Catalytic cracking has undergone progressive development since the 40s. The trend of development of the fluid catalytic cracking (FCC) process has been to all riser cracking and use of zeolite catalysts.
Riser cracking gives higher yields of valuable products than dense bed cracking. Most FCC units now use all riser cracking, with hydrocarbon residence times in the riser of less than 10 seconds, and even less than 5 seconds, and in some cases less than 1 second.
Zeolite-containing catalysts having high activity and selectivity are now used in most FCC units. These catalysts work best when coke on the catalyst after regeneration is less than 0.1 wt %, and preferably less than 0.05 wt %.
To regenerate FCC catalysts to these low residual carbon levels, and to burn CO completely to CO.sub.2 within the regenerator (to conserve heat and minimize air pollution) many FCC operators have turned to high efficiency regenerators and to CO combustion promoters.
High efficiency regenerators were a breakthrough in catalyst regeneration in FCC. Rather than rely on a large, single dense phase, bubbling fluidized bed, catalyst regeneration was conducted in a new device containing a fast fluidized bed, a dilute phase transport riser, and a second dense bed.
In the fast fluidized bed, sometimes called a coke combustor, stripped catalyst, usually along with some recycled, hot, regenerated catalyst, was substantially decoked by contact with air. Superficial vapor velocities in the coke combustor were high, usually near the upper limit of velocity where a dense phase fluidized bed could be maintained. The coke combustor operated without large bubbles, which had plagued the operation of the large, single dense bed regenerators previously used.
The dilute phase transport riser was above the coke combustor. A gradual reduction in cross sectional area of a transition section between the coke combustor and the transport riser increased the vapor velocity, and led to a dilute phase regime in the upper portion of the coke combustor and in the transport riser. CO combustion to CO.sub.2 proceeded rapidly in the dilute phase.
The second dense bed served to collect regenerated catalyst discharged from the transport riser. The collected regenerated catalyst was recycled to the cracking reactor, and usually a large amount was recycled to the coke combustor to heat the incoming stripped catalyst and promote rapid coke combustion in the coke combustor, and CO combustion in the transport riser.
Most FCC operators, with both the old and the new or high efficiency regenerators, added a CO combustion promoter.
U.S. Pat. Nos. 4,072,600 and 4,093,535, which are incorporated by reference, teach use of combustion-promoting metals such as Pt, Pd, Ir, Rh, Os, Ru and Re in cracking catalysts in concentrations of 0.01 to 50 ppm, based on total catalyst inventory.
As the process and catalyst improved, refiners attempted to use the process to upgrade a wider range of feedstocks, in particular, feedstocks that were heavier, and also contained more metals and sulfur than had previously been permitted in the feed to a fluid catalytic cracking unit.
These heavier, dirtier feeds have placed a growing demand on the regenerator. Processing resids has exacerbated many existing problem areas in the regenerator, and caused the coke burning capacity of the regenerator to be the limiting factor in some FCC units.
With heavier feeds, more coke is often deposited on the catalyst than is needed for the cracking reaction. The regenerator gets hotter, and the extra heat is rejected as high temperature flue gas. Many refiners severely limit the amount of resid or similar high CCR feeds to that amount which can be tolerated by the unit. High temperatures are a problem for the metallurgy of many units, but more importantly, are a problem for the catalyst. In the regenerator, the burning of coke and unstripped hydrocarbons leads to much higher surface temperatures on the catalyst than the measured dense bed or dilute phase temperature. This is discussed by Occelli et al in Dual-Functi Cracking Catalyst Mixtures, Ch. 12, Fluid Catalytic Cracking, Symposium Series 375, American Chemical Society, Washington, D.C., 1988.
Some regenerator temperature control is possible by adjusting the CO/CO.sub.2 ratio produced in the regenerator. Burning coke partially to CO produces less heat than complete combustion to CO.sub.2. However, in some cases, this control is insufficient, and also leads to increased CO emissions, which can be a problem unless a CO boiler is present.
U.S. Pat. No. 4,353,812 to Lomas et al, which is incorporated by reference, discloses cooling catalyst from a regenerator by passing it through the shell side of a heat-exchanger with a cooling medium through the tube side. The cooled catalyst is recycled to the regeneration zone. This approach will remove heat from the regenerator, but will not prevent poorly, or even well, stripped catalyst from experiencing very high surface or localized temperatures in the regenerator.
The prior art also used dense or dilute phase regenerated fluid catalyst heat removal zones or heat-exchangers that are remote from, and external to, the regenerator vessel to cool hot regenerated catalyst for return to the regenerator. Examples of such processes are found in U.S. Pat. Nos. 2,970,117 to Harper; 2,873,175 to Owens; 2,862,798 to McKinney; 2,596,748 to Watson et al; 2,515,156 to Jahnig et al; 2,492,948 to Berger; and 2,506,123 to Watson.
Although catalyst coolers, or limiting operation to partial CO combustion will help some, these methods alone will not get around that fact that the coke burning capacity of many high efficiency regenerators is finite. Even putting in more air blowers, and putting myriad catalyst coolers around the regenerator, will not bypass the somewhat limited coke burning capacity of the coke combustor.
I have found a way to significantly increase the coke burning capacity of high efficiency FCC catalyst regenerators. My method can also be used to achieve somewhat cleaner catalyst in these regenerators. I realized that these regenerators suffered from an oversight, namely that much of the regenerator sat idle and performed little or no useful work. I realized that a significant increase in coke burning capacity could be achieved by using a heretofore neglected approach to catalyst regeneration in these units, specifically, adding some of the spent catalyst to be regenerated to the second dense bed, instead of adding all the spent catalyst to the coke combustor. By using a conventional high efficiency regenerator in an unconventional way, and bypassing the coke combustor, I can increase the coke burning capacity of these units by preheating some of the stripped catalyst in the second dense bed, by regenerating some of the stripped catalyst in the second dense bed, or both.